Heavy metals such as cadmium and lead contained in natural substances such as minerals and soil have been discharged into the environment by human activities such as mine development.
For example, cadmium is contained in mine water, discharged water or smoke from a smelter, and wastewater from gob deposits, and such cadmium is a major cause of soil contamination at present. Also, cadmium is imported and produced in Japan, and it has been used for nickel-cadmium batteries, pigments, alloys, and a polyvinyl chloride stabilizer in the past. In the past, it was discharged into rivers and the air as discharged water or smoke from factories and incineration plants. Further, it is known that cadmium invades soil in croplands with the incursion from river water or the environment such as through rain water that has been contaminated with cadmium.
Some cadmium contained in food is absorbed by and accumulated in the body upon food ingestion. Accordingly, the Ministry of Health, Labor and Welfare has announced that long-term ingestion of food with high cadmium content may cause renal dysfunction. In addition, cases of itai-itai disease have been caused by environmental cadmium contamination in Japan.
The international standards for food cadmium content have been under review by the Codex Alimentarius Commission since 1998. The Codex Alimentarius Commission announced international standards in July 2007, such as 0.4 mg of cadmium per kg of polished rice and 0.2 mg thereof per kg of wheat.
The cadmium content in crops can be reduced via, for example, introduction of soil dressing or soil improvement agents, utilization of a plant variety having a low capacity for cadmium absorption, or soil cleanup via phytoremediation or soil washing.
At present, soil improvement is mainly carried out by a method involving soil dressing. This method, however, requires tremendous amounts of money, and it is difficult to collect the amount of mountain soil required for soil dressing. In addition, soil dressing necessitates the disposal of large quantities of waste soil. Further, it is necessary to improve soil so that it will be suitable for crop cultivation. When the depth of soil dressing is insufficient, contamination may occur again. Thus, development of a technique that is sufficient in respect of physical and economical conditions has been awaited. Other techniques also require tremendous amounts of money and time.
Removal of cadmium from the environment with the aid of microorganisms is expected as a cost-effective technique. Metal cations are bound to anions, such as sulfide ions or hydroxide ions, and converted into hardly-soluble compounds in water. Sulfides, hydroxides, phosphates, and the like of heavy metal ions are insoluble or hardly-soluble in water. With the utilization of such properties, removal of cadmium from the environmental water has been attempted using microorganisms. As microorganisms that precipitate sulfides, sulfate-reducing bacteria of the genus Desulfovibrio and the like are known. It is known that such microorganisms are distributed in lakes, wetlands, or rice paddies in which large quantities of organic matter exist, and research regarding the reduction of heavy metals in paddy rice has been conducted. While such microorganisms are effective for rice cultivated in rice paddies, sulfides are converted into sulfuric acids, which are absorbable by crops in fields under acidic conditions. Thus, such microorganisms cannot be used in fields under acidic conditions.
It was reported that the cadmium concentration in the roots of garden peas was lowered via treatment with the use of mycorrhizal fungi. Mycorrhizal fungus is known to infect roots of a specific host and accelerate the nutrient absorption of plants, and particularly phosphoric acid absorption. However, the effects of cadmium reduction were not observed in soil containing highly concentrated cadmium (Journal of Experimental Botany, 2002, 53, pp. 1177-1185).
Also, an attempt to reduce cadmium in tobacco with the use of mycorrhizal fungi of the genus Glomus was reported (Applied Soil Ecology, 2007, 35, pp. 502-510). The effects of cadmium reduction in leaves were observed in 3 Arbuscular Mycorrhiza (AM). However, no correlation has been observed between the effects of cadmium reduction and colonization of mycorrhizal fungi. AM are mold species, which are difficult to artificial culture, and mass culture thereof is laborious and time-consuming. Further, mycorrhizal fungi only colonize roots of a specific host and it is difficult to use them except their host. Functions of mycorrhizal fungi are known to differ significantly depending on the soil, and the functions thereof are known to be suppressed in soil that is rich in phosphoric acid or soil that is poor in air permeability. Thus, the effects of cadmium reduction may significantly vary depending on soil. Also, the experiment of Applied Soil Ecology, 2007, 35, pp. 502-510 was performed only in a greenhouse, and soil immediately after sterilization was used. When sterilized soil is used, it is deduced that microorganisms added via inoculation become the dominant species in such soil. When plants are cultivated, in general, a wide variety of microorganisms exist in soil, and the amounts of inoculated microorganisms decrease via nutrient competition, for example. Thus, it is important to conduct an experiment under environments in which plants are actually cultivated, such as in fields.
Bacteria have been heretofore used as control agents of soil-borne diseases or plant diseases.
For example, Ochrobactrum anthropi TRB19 strain is used as a control agent of a soil disease for Solanaceous plants, and this strain exhibits the effects of controlling bacterial wilt disease against Solanaceous plants including tobacco and tomato upon application thereof to roots, cultivation area, and/or soil used for cultivation of Solanaceous plants (JP Patent Publication (kokai) No. H09-194314 A (1997)).
Alcaligenes faecalis (Accession Number: FERM P-15229) is used as a control agent of a plant disease for cucumbers, and it is used for a method in which the culture product of such bacteria or bacteria cell are suspended in water and the resulting suspension is sprayed on plants or a method in which it is sprayed on foliage aimed at controlling foliar diseases, such as powdery mildew (JP Patent Publication (kokai) No. H09-104606 A (1997)).
A bacterial strain of the genus Variovorax (CGF4526, Accession Number: FERM P-19563) is used as a control agent for the clubroot disease of Brassicaceae plants, and such control agent comprising the strain is mixed with soil or subjected to perfusion of soil when seedlings are raised, or seedlings are soaked in the control agent before being planted in the field (JP Patent Publication (kokai) No. 2005-137330 A).
In addition, a combination of the bacterial strain of the genus Variovorax (CGF4526) and nonpathogenic Erwinia carotovora subsp. carotovora is used as a control agent for the clubroot disease of Brassicaceae plants, and such control agent is applied when raising seedlings (i.e., before transplantation to the main field) (JP Patent Publication (kokai) No. 2007-197421 A).